The present invention generally relates to a hydrogen permeable structure and a method of manufacturing the structure, and more particularly to a hydrogen permeable structure in which a hydrogen permeable film is formed in a porous substrate, a method of manufacturing the structure, and a method of repairing the structure.
A hydrogen gas is used as, e.g., fuel for fuel cells and is industrially manufactured by, e.g., a gaseous fuel denaturing process. With the gaseous fuel denaturing process, for example, a hydrogen gas is manufactured by denaturing water vapor. A denatured gas contains, in addition to hydrogen as a primary component, carbon monoxide and carbon dioxide as secondary components. Direct use of such a denatured gas as, e.g., fuel for fuel cells deteriorates cell performance. It is therefore required to remove the secondary components other than the hydrogen gas, and to refine the denatured gas for obtaining a high-purity hydrogen gas. One of known refining methods utilizes a characteristic in which a hydrogen permeable film selectively allows only hydrogen to pass through the film. The hydrogen permeable film is formed on a porous support or substrate when used.
For example, Japanese Unexamined Patent Application Publication No. 11-267477 proposes a hydrogen permeable structure in which a hydrogen permeable film, such as a Pd film or a Nb film, having a thickness of about 0.1 to 20 xcexcm is formed by an ion plating process on the surface of a porous support made of stainless steel or ceramic, e.g., alumina or silicon nitride.
Also, Japanese Unexamined Patent Application Publication No. 11-286785 proposes a hydrogen permeable structure in which a Pd metal and a metal capable of alloying with Pd are alternately multi-layered on the surface of a porous support by an electroless plating process or an ion plating process, and the multi-layers are subjected to heat treatment to form a Pd alloy film as a hydrogen permeable film.
Further, Japanese Unexamined Patent Application Publication No. 4-349926 proposes a hydrogen gas separation film in which pores of an inorganic porous body with pore sizes of 10 to 10000 xc3x85 support therein silica gel having an average pore size of 10 to 30 xc3x85, alumina gel having an average pore size of 15 to 30 xc3x85, or silica-alumina gel having an average pore size of 10 to 20 xc3x85, and a thin film containing palladium is formed as a hydrogen permeable film on the surface of the porous body.
Each of the above-mentioned publications discloses the structure in which the hydrogen permeable film is formed on the surface of the porous support. However, when those hydrogen permeable structures were used in an atmosphere under various conditions, problems occurred in which the hydrogen permeable film peeled off and durability was poor.
As one example of techniques for depositing Pd on a non-metallic material such as a ceramic, electroless plating using sodium phosphite (NaH2PO3) as a reductant is disclosed in xe2x80x9cHyomen Gijutsu (Surface Technology)xe2x80x9d, 42, 1146(1991). With this disclosed technique, however, it was impossible to freely control the plating position. Further, U.S. Pat. No. 5,789,027 discloses a method of depositing a Pd on a substrate, that is, a method in which a Pd compound is dissolved together with a hydrogen gas in a supercritical fluid of CO2 so as to be supplied onto the substrate, thereby depositing Pd on the substrate. However, this disclosed method requires a fluid in the supercritical state and is not economical.
An object of the present invention is to provide a hydrogen permeable structure which can effectively prevent peeling-off of a hydrogen permeable film and hence has higher durability, and to a method of manufacturing the structure. According to the method of the present invention, the position where a thin film containing Pd is to be formed can be controlled as desired, no special technique such as a supercritical fluid is required, and defects such as pinholes can easily be repaired.
The present invention has been accomplished based on the finding that, by forming thin films within pores of a porous support in shapes corresponding to individual pore shapes, a hydrogen permeable structure being highly resistant to peeling-off of the thin films and having superior durability can be obtained because peripheries of the thin films are supported by a skeleton of the support.
According to the present invention, by supplying a solution containing Pd through one surface of a porous support and supplying a solution containing a reductant through the other surface of the porous support, the solution containing Pd and the solution containing the reductant contact with each other on the surface of or inside the porous support, whereby the Pd is reduced and metallic Pd is deposited. Therefore, thin films containing Pd can be formed on the surface of the porous support and within pores in the surface of the porous support or within pores inside the porous support.
Alternatively, by supplying a reducing gas instead of the solution containing the reductant, metallic Pd is also precipitated by means of reduction of Pd as described above such that thin films containing Pd can be formed on the surface of the porous support or within pores inside the porous support. Deposition of metallic Pd continues as long as the solution containing Pd contacts the solution containing the reductant or the reducing gas. In other words, reduction reaction continues until the pores of the porous support are sealed off by Pd.
In the case using the reducing gas, by filling a material permeable to the reducing gas within the pores of the porous support, a thin film containing Pd can be formed on an end surface of the reducing-gas permeable material. Therefore, the Pd thin film can be formed at a desired position within the porous support.
The solution containing Pd is not limited to a particular one provided that the solution contains palladium. Examples of such solution include a solution of a Pd complex ion in which ligands, such as NO2 and NH3, are coordinated in number not less than two and not more than six, and a solution of palladium chloride or palladium nitrate. Also, the solution containing Pd is preferably a solution containing chlorine and palladium. Further, the solution containing Pd is preferably a solution containing platinum as well as chlorine and palladium. A hydrogen permeable film containing Pd to which Pt is added has less solubility to hydrogen at a predetermined temperature than that containing Pd alone. Therefore, an amount of expansion of the crystal lattice of a palladium metal, i.e., an amount of expansion of the film, can be suppressed. It is hence possible to reduce compressive stresses caused in the film upon expansion thereof, and hence to reduce stresses imposed on the interface between the film and a substrate. As a result, physical deterioration of the hydrogen permeable film, such as peeling-off and cracks, can be greatly reduced.
The solution containing the reductant is, for example, a solution containing, as a reductant, a phosphate or a hypophosphite, e.g., H2PO2xe2x88x92 or HPO32xe2x88x92, hydrazine, formaldehyde, dimethylamine borane, or any of tetrahydra borates such as NaBH4, LiBH4 and KBH4. Preferably, the solution containing the reductant is an alcoholic or aqueous solution in which at least one of those reductants is dissolved.
By spraying with a sprayer either or both of the solution containing Pd and the solution containing the reductant in a state of mist, for example, disturbance at the interface between the Pd-containing solution and the reductant-containing solution is reduced and the pores can be sealed off with thinner films. Accordingly, the spraying method is able to reduce the amount of deposited Pd to a value not more than 5 g/m2 and is more economical. Herein, the term xe2x80x9camount of deposited Pdxe2x80x9d represents a value normalized with respect to a Pd deposited area regardless of shape of the hydrogen permeable structure. More specifically, an area of 1 to 10 cm2 of the hydrogen permeable structure, in which Pd has been deposited, is cut out from any desired position, and a cut-out specimen is dissolved in an acid. The Pd concentration of the thus obtained solution is analyzed with plasma emission spectroscopic analysis to calculate a total amount of Pd. Then, the amount of deposited Pd is obtained by dividing the total amount of Pd by the area of the specimen.
The reducing gas is preferably a hydrogen gas, but any other suitable gas may be mixed in a hydrogen gas for control of the reaction velocity. Further, the gas permeable material is preferably paraffin. Paraffin is permeable to hydrogen and can be dissolved and removed with an organic solvent, such as dichloromethane.
Preferably, thin films formed inside the porous support and containing Pd have an average thickness of not more than 2 xcexcm and not less than 0.01 xcexcm. Also, thin films formed on the surface of the porous support and containing Pd have an average thickness of not more than 2 xcexcm and not less than 0.01 xcexcm. In the thin films formed inside or on the surface of the porous support and containing Pd, the deposition rate of Pd is preferably not more than 5 g/m2. The porous support is preferably a porous body of silicon nitride or a metallic porous body.
Further, the porous support has holes in the surface thereof, and preferably is provided with a porous oxide layer, or a layer of metal or metal oxide having an average particle size of not more than 2 xcexcm such that the holes are covered therewith. With such structure, since the holes in the surface of the porous support are filled, making the surface even, the hydrogen permeable film can be formed in a dense state free from pinholes when it is formed on the surface of the porous support. Accordingly, the permeability characteristics of the hydrogen permeable film can be improved. In such a case, preferably, the oxide layer contains at least one selected from the group consisting of aluminum oxide (Al2O3), silicon dioxide (SiO2) and zirconium oxide (ZrO2). The oxide layer is more preferably made of aluminum oxide.
In the hydrogen permeable structure in which a Pd-containing thin film has been formed on the surface of or inside the porous support thereof, defects of the Pd-containing thin film, such as pinholes, can easily be repaired by supplying the solution containing Pd onto one surface of the hydrogen permeable structure and supplying the solution containing the reductant onto the other surface of the hydrogen permeable structure, since thereby a metal containing Pd can be deposited in the pinholes with priority. In that case, a similar effect is also obtained by using a reducing gas instead of the solution containing the reductant.
If a hydrogen permeable structure includes a layer in which pores are sealed off by depositing a metal containing Pd in a porous support and/or porous powder, its durability can be further improved. In such a hydrogen permeable structure, an amount of nitrogen permeable through the structure can be reduced to a level not more than 0.6 ml/min/cm2 under a differential pressure of 1 atmospheric pressure. As a result, hydrogen having a higher purity can be obtained.
Additionally, a Pd-containing film can be made denser by performing heat treatment of the Pd-containing film in a non-oxidizing atmosphere, e.g., a vacuum, a nitrogen atmosphere or a hydrogen atmosphere, after the Pd-containing film has been formed.